<!DOCTYPE html>
<!-- 需要添加
<html  xmlns:th="http://www.thymeleaf.org">
这样在后面的th标签就不会报错
 -->
<html  xmlns:th="http://www.thymeleaf.org">
<head lang="en">
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
    <title th:text="${username}">xx</title>
</head>
<body>
<h1 th:text="${username}">Hello World</h1>
</body>
</html>